To add to the costs:
We had a 50m^3 loft conversion with two hip to gables and a large rear dormer put in in 2013. The conversion bit was £60k. It didn't include supply of the bathroom suite or decorating. Everything else was included (building works, scaffolding, skips, building control, electrics, windows, fitting bathroom suite etc).
We are in zone 5 Barnet. Lots of free parking etc.